1. Field of the Invention
This invention relates to a method for forming an energy subtraction image wherein, from a plurality of radiation images, an energy subtraction image is formed which includes little noise and which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness. This invention also relates to a superposition processing method and apparatus for radiation images, wherein an addition process is carried out on two image signals representing radiation images of a single object.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal, and the electric signal (image signal) is processed and then used for reproducing the X-ray image as a visible image on a photocopy, or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like, can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays, such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT), or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order for the desired image density to be obtained, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal (image signal) to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT.
In the radiation image recording and reproducing systems wherein recording media, such as X-ray film or stimulable phosphor sheets, are used, subtraction processing techniques for radiation images are often carried out on image signals detected from a plurality of radiation images of an object which have been recorded on the recording media.
With the subtraction processing techniques for radiation images, an image is obtained which corresponds to a difference between a plurality of radiation images of an object recorded under different conditions. Specifically, a plurality of the radiation images recorded under different conditions are read out at predetermined sampling intervals, and a plurality of image signals thus detected are converted into digital image signals which represent the radiation images. The image signal components of the digital image signals which represent the image information recorded at corresponding sampling points in the radiation images are then subtracted from each other. A difference signal is thereby obtained which represents the image of a specific structure or part of the object represented by the radiation images.
Basically, subtraction processing is carried out with either the so-called temporal (time difference) subtraction processing method or the so-called energy subtraction processing method. In the former method, in order for the image of a specific structure (for example, a blood vessel) of an object to be extracted from the image of the whole object, the image signal representing a radiation image obtained without injection of contrast media is subtracted from the image signal representing a radiation image in which the image of the specific structure (for example, a blood vessel) of the object is enhanced by the injection of contrast media. In the latter method, such characteristics are utilized that a specific structure of an object exhibits different levels of radiation absorptivity with respect to radiation with different energy levels. Specifically, an object is exposed to several kinds of radiation with different energy levels, and a plurality of radiation images are thereby obtained in which different images of a specific structure are embedded. Thereafter, the image signals representing the plurality of the radiation images are weighted appropriately and subjected to a subtraction process in order to extract the image of the specific structure. The applicant proposed novel energy subtraction processing methods using stimulable phosphor sheets in, for example, U.S. Pat. Nos. 4,855,598 and 4,896,037.
A plurality of radiation images, which are subjected to energy subtraction processing, will herein be referred to as the "original images". An image signal representing a subtraction image is obtained by subtracting the image signals representing the original images from each other. Therefore, the image signal representing the subtraction image has a lower signal-to-noise ratio (S/N ratio) than the image signals representing the original images. As a result, the problems occur in that the image quality of the subtraction image becomes worse than the image quality of the original images.
By way of example, energy subtraction processing is often carried out in the manner described below. Specifically, an object, such as the chest of a human body, which is constituted of soft tissues and bones, is exposed to several kinds of radiation with different energy levels, and a plurality of radiation images of the object are thereby obtained. The plurality of the radiation images are read out, and a plurality of image signals representing the radiation images are generated. Energy subtraction processing is then carried out on the plurality of the image signals. From the energy subtraction processing, a soft tissue image signal is obtained which represents a soft tissue image primarily composed of patterns of the soft tissues of the object. Alternatively, a bone image signal is obtained which represents a bone image primarily composed of patterns of the bones of the object. Thereafter, the soft tissue image is reproduced as a visible image from the soft tissue image signal, or the bone image is reproduced as a visible image from the bone image signal. In the soft tissue image, the patterns of the bones have been erased. Therefore, patterns, which were behind the bone patterns or were rendered imperceptible by the bone patterns in the original images, become more perceptible in the soft tissue image than in the original images. Also, in the bone image, the patterns of the soft tissues have been erased. Therefore, patterns, which were behind the soft tissue patterns or were rendered imperceptible by the soft tissue patterns in the original images, become more perceptible in the bone image than in the original images. Accordingly, a subtraction image can be obtained which is well matched to the purposes of diagnosis. However, because the soft tissue image and the bone image are obtained from the subtraction processing, the problems occur in that noise components have been emphasized in the soft tissue image and the bone image than in the original images. From this point of view, the image quality of the soft tissue image and the bone image could not heretofore been kept good.
Accordingly, in pending U.S. patent application Ser. No. 654,450, the applicant proposed a method for forming an energy subtraction image wherein a subtraction image is formed in which noise has been reduced. The proposed method comprises the steps of:
i) after a plurality of radiation images of an object are recorded on recording media by irradiating several kinds of radiation with different energy levels to the object, which is constituted of a plurality of tissues exhibiting different levels of radiation absorptivity with respect to the several kinds of radiation with different energy levels, and a plurality of original image signals representing the plurality of the radiation images are then detected, PA1 generating a first image signal, which represents a first image primarily composed of patterns of first tissues of the object, from the plurality of the original image signals, PA1 ii) generating a first smoothed image signal by processing the first image signal, the first smoothed image signal representing a first smoothed image in which noise components of the first image have been reduced or eliminated, and PA1 iii) generating a second image signal by subtracting the first smoothed image signal from an original image signal, the second image signal representing a second image primarily composed of patterns of second tissues of the object. PA1 a) generating a second smoothed image signal by processing the second image signal, the second smoothed image signal representing a second smoothed image in which noise components of the second image have been reduced, and PA1 b) generating a new first image signal by subtracting the second smoothed image signal from an original image signal, the new first image signal representing a new first image primarily composed of the patterns of the first tissues of the object. PA1 i) after two kinds of radiation images of an object are formed with two kinds of radiation having different energy levels, the object being constituted of a plurality of tissues exhibiting different levels of radiation absorptivity with respect to the two kinds of radiation having different energy levels, PA1 ii) generating a superposition image signal by carrying out a weighted addition expressed as EQU S=(N2.sup.2 /(N1.sup.2 +N2.sup.2)).times.SO1+(N1.sup.2 /(N1.sup.2 +N2.sup.2)).times.SO2 PA1 wherein SO1 represents the first original image signal, N1 represents noise contained in the first original image signal, SO2 represents the second original image signal, and N2 represents noise contained in the second Original image signal, PA1 iii) generating a first image signal, which represents a first image primarily composed of patterns of first tissues of the object, by subtracting the first original image signal and the second original image signal from each other, PA1 iv) generating a first smoothed image signal by smoothing the first image signal, the first smoothed image signal representing a first smoothed image in which noise components of the first image have been reduced or eliminated, and PA1 v) generating a second image signal by subtracting the superposition image signal and the first smoothed image signal from each other, the second image signal representing a second image primarily composed of patterns of second tissues of the object. PA1 i) after two kinds of radiation images of an object are formed with two kinds of radiation having different energy levels, the object being constituted of a plurality of tissues exhibiting different levels of radiation absorptivity with respect to the two kinds of radiation having different energy levels, PA1 detecting first and second original image signals representing the two kinds of the radiation images, PA1 ii) generating a superposition image signal by carrying out a weighted addition expressed as EQU S=(N2.sup.2 /(N1.sup.2 +N2.sup.2)).times.SO1+(N1.sup.2 /(N1.sup.2 +N2.sup.2)).times.SO2 PA1 wherein SO1 represents the first original image signal, N1 represents noise contained in the first original image signal, SO2 represents the second original image signal, and N2 represents noise contained in the second original image signal, PA1 iii) carrying out a first process for generating a first image signal, which represents a first image primarily composed of patterns of first tissues of the object, by subtracting the first original image signal and the second original image signal from each other, PA1 iv) thereafter carrying out a second process, which comprises the steps of: PA1 v) thereafter carrying out a third process, which comprises the steps of: PA1 i) carrying out the processes in the second method for forming an energy subtraction image in accordance with the present invention, and PA1 ii) thereafter repeating the following once or several times: PA1 i) carrying out the processes in the second or third method for forming an energy subtraction image in accordance with the present invention, and PA1 ii) generating a new second image signal by carrying out the second process or the new second process in which the new first image signal obtained from the third process or the new third process is taken as the first image signal in the second process or the new second process, the new second image signal thus most recently generated representing a new second image primarily composed of the patterns of the second tissues of the object. PA1 carrying out a weighted addition on the image signal components of the first original image signal and the second original image signal, which represent corresponding picture elements in the two kinds of the radiation images, the weighted addition being expressed as EQU S=(Sk2/(Sk1+Sk2)).times.SO1+(Sk1/(Sk1+Sk2)).times.SO2 PA1 calculating a sensitivity Sk1 of each of image signal components of the first original image signal, which represent picture elements in the corresponding radiation image, and a sensitivity Sk2 of each of image signal components of the second original image signal, which represent picture elements in the corresponding radiation image, on the basis of the unsharp signal of the first original image signal SO1 and the unsharp signal of the second original image signal SO2, and PA1 carrying out a weighted addition on the image signal components of the first original image signal and the second original image signal, which represent corresponding picture elements in the two kinds of the radiation images, the weighted addition being expressed as EQU S=(Sk2/(Sk1+Sk2)).times.SO1+(Sk1/(Sk1+Sk2)).times.SO2 PA1 the superposition processing method for radiation images comprising generating the addition signal by carrying out a weighted addition expressed as EQU S=(N2.sup.2 /(N1.sup.2 +N2.sup.2)).times.S1+(N1.sup.2 /(N1.sup.2 +N2.sup.2)).times.S2 PA1 wherein the addition means carries out a weighted addition expressed as EQU S=(N2.sup.2 /(N1.sup.2 +N2.sup.2)).times.S1+(N1.sup.2 /(N1.sup.2 +N2.sup.2)).times.S2 PA1 the superposition processing method for radiation images comprising generating the addition signal by carrying out a weighted addition expressed as EQU S=(Sk2/(Sk1+Sk2)).times.S1+(Sk1/(Sk1+Sk2)).times.S2 PA1 wherein the addition means carries out a weighted addition expressed as EQU S=(Sk2/(Sk1+Sk2)).times.S1+(Sk1/(Sk1+Sk2)).times.S2 PA1 the superposition processing method for radiation images comprising generating the addition signal S by the steps of: PA1 i) calculating a sensitivity Sk1 of each of image signal components of the first image signal, which represent picture elements in the corresponding radiation image, and a sensitivity Sk2 of each of image signal components of the second image signal, which represent picture elements in the corresponding radiation image, on the basis of the first image signal S1 and the second image signal S2, and PA1 ii) carrying out a weighted addition on the image signal components of the first image signal and the second image signal, which represent corresponding picture elements in the two kinds of the radiation images, the weighted addition being expressed as EQU S=(Sk2/(Sk1+Sk2)).times.S1+(Sk1/(Sk1+Sk2)).times.S2 PA1 wherein the addition means comprises: PA1 i) an operation device for calculating a sensitivity Sk1 of each of image signal components of the first image signal, which represent picture elements in the corresponding radiation image, and a sensitivity Sk2 of each of image signal components of the second image signal, which represent picture elements in the corresponding radiation image, on the basis of the first image signal S1 and the second image signal S2, and PA1 ii) an addition device for carrying out a weighted addition on the image signal components of the first image signal and the second image signal, which represent corresponding picture elements in the two kinds of the radiation images, the weighted addition being expressed as EQU S=(Sk2/(Sk1+Sk2)).times.S1+(Sk1/(Sk1+Sk2)).times.S2 PA1 the superposition processing method for radiation images comprising generating the addition signal S by the steps of: PA1 i) generating an unsharp signal of the first image signal S1 and an unsharp signal of the second image signal S2, PA1 ii) calculating a sensitivity Sk1 of each of image signal components of the first image signal, which represent picture elements in the corresponding radiation image, and a sensitivity Sk2 of each of image signal components of the second image signal, which represent picture elements in the corresponding radiation image, on the basis of the unsharp signal of the first image signal S1 and the unsharp signal of the second image signal S2, and PA1 iii) carrying out a weighted addition on the image signal components of the first image signal and the second image signal, which represent corresponding picture elements in the two kinds of the radiation images, the weighted addition being expressed as EQU S=(Sk2/(Sk1+Sk2)).times.S1+(Sk1/(Sk1+Sk2)).times.S2 PA1 wherein the addition means comprises: PA1 i) an operation device for generating an unsharp signal of the first image signal S1 and an unsharp signal of the second image signal S2, and PA1 calculating a sensitivity Sk1 of each of image signal components of the first image signal, which represent picture elements in the corresponding radiation image, and a sensitivity Sk2 of each of image signal components of the second image signal, which represent picture elements in the corresponding radiation image, on the basis of the unsharp signal of the first image signal S1 and the unsharp signal of the second image signal S2, and PA1 ii) an addition device for carrying out a weighted addition on the image signal components of the first image signal and the second image signal, which represent corresponding picture elements in the two kinds of the radiation images, the weighted addition being expressed as EQU S=(Sk2/(Sk1+Sk2)).times.S1+(Sk1/(Sk1+Sk2)).times.S2
In pending U.S. patent application Ser. No. 654,450, the applicant also proposed a method for forming an energy subtraction image wherein, after the aforesaid first process is carried out in order to generate the first image signal, the second process is carried out in order to generate the second image signal. Thereafter, a third process is carried out, which comprises the steps of:
The proposed method for forming an energy subtraction image may be modified such that the second process or the third process is repeated, and noise components of the image signal may be reduced even further.
With the methods for forming an energy subtraction image, which are proposed in pending U.S. patent application Ser. No. 654,450, the image signal components of the first image signal and the second image signal are added to each other which represent the image information stored at corresponding picture elements in the two radiation images. In this manner, a superposition image signal SO is obtained, which can be expressed as EQU SO=(SO1+SO2)/2
wherein SO1 represents the first image signal, and SO2 represents the second image signal. In the superposition image signal SO, noise components of the image signal have been reduced. Therefore, the addition process is advantageous for the subsequent processes.
However, the superposition image signal is obtained with the formula shown above wherein the values of the first image signal and the second image signal are merely averaged. Therefore, the degree of reduction in the noise components is not necessarily be appropriate. Thus a need exists for a method for forming an energy subtraction image, which yields an energy subtraction image in which the noise components have been reduced even further.